Tuesday, January 29, 2008

Things that keep me up at night..

I've tried to cut back on the amount of personal stuff that I publish here, and limit it just to professional topics. So, no more discussions about politics, what I ate for breakfast or drunken debauchery.

This, however, rides the fine line between personal and professional. The following three things make me lose sleep

  1. Everyone's favorite layer 2 protocol, ethernet, has the destination MAC address before the source MAC address. I've heard and speculated that this is for speed. Instead of having to jump ahead another 6 bytes, small devices that have limited processing power can read in just 6 bytes and be able to make forwarding decisions accordingly. Why, though, does this practice only exist at this layer? Sure, the "source then destination" thing is burned into our brain, but what performance gains could be obtained had, say, destination IP addresses been placed earlier in IPv4?
  2. Why are UDP and TCP checksums so crazily different than, say, IP or ICMP checksums? A checksum exists so that protocols that decide to implement it can be sanity checked -- essentially, has this layer and the stuff it is responsible for been damaged in transit? IPv4, not caring about the payload, only checksums its header values. It is up to higher level protocols to check themselves before they wreck themselves. ICMP takes a slightly more polite approach and checksums its header values and payload. UDP and TCP, however, take this grossly different approach -- in addition to computing a checksum that uses their respective header values and payloads, these two decide to also include the source and destination IP addresses in the checksum. WTF? A protocol should be able to compute its own checksum by utilizing whatever information is available to it. This includes its header values and whatever is in its payload, if anything. Data lower in the stack is simply not available in any sort of simplistic fashion, yet TCP and UDP decide to be special.

1 comment:

Ken Dreyer said...

There's only two things